NEW PALEOMAGNETIC DATA FROM UPPER PERMIAN/LOWER TRIASSIC VOLCANIC SEQUENCES AND CRETACEOUS RED BEDS FROM NORTHERN VIETNAM--THEIR BEARING ON THE ACCRETION HISTORY OF SOUTHEAST ASIA AND ORTHOGONAL DEMAGNETIZATION DIAGRAMS KNOWN AND LOVED

New paleomagnetic data from Upper Permian to Lower Triassic volcanic rocks from NW Vietnam and Cretaceous red beds and volcanic rocks in NW and NE Vietnam further refine the paleogeographic setting of crustal elements of the Song Da Terrane (NW Vietnam), east of the Song Ma suture, between the South China block (SCB) and north Indochina and the inferred southernmost SCB (NE Vietnam). These include results from 12 sites (125 samples) from basalts of the Vien Nam Fm, Hoa Binh Dam; eight sites (74 samples) from basalts of the Cam Thuy Fm near Thuan Chau; 19 sites (198 samples) in andesites and basalts of the Vien Nam Fm near Quynh Nhai; and over 1000 samples collected from red beds at over 10 localities. The collection is limited by the quality of exposures and quantity of independent flows or distinct beds. Most volcanic rocks yield interpretable magnetizations in progressive demagnetization; the response implies that characteristic remanent magnetization (ChRM) components are carried by low-titanium magnetite or hematite, or a combination of both; these are isolated from secondary components. Rock magnetic data and petrography support the retention of an early-acquired thermoremanent magnetization; anisotropy of magnetic susceptibility (AMS) are not consistent with a pervasive post-emplacement deformation fabric. Red beds with interpretable ChRM (of dual polarity) are dominated by hematite as the principal carrier. Permian and Triassic volcanic rocks yield data implying formation at shallow south latitudes. Flows at Quynh Nhai likely yield the most robust paleofield; the data set is of dual polarity, passing a reversal test. The tilt corrected grand mean (normal polarity) is D= 48.3^o, I=-10.0^o, a₉₅=8.0^o, k=27.7, N=13, yielding an inferred paleomagnetic pole at lat=35.7N, long=217.4E, and a paleolatitude of 5.1^oS. Compared with the Late Permian-Early Triassic SCB apparent polar wander path, the data show that crustal elements of NW Vietnam have been close to, but not unequivocally a coherent part of, the SCB, since the Late Permian. Development of the parallel NW-SE striking Song Ma and Song Chay orogenic belts did not involve the closure of wide (> 500 km) ocean basins. Despite variations in declination among localities, inclination data from red beds do not show appreciable latitudinal discordance with the SCB.